The present invention relates generally to a liquid crystal display, and more particularly to a liquid crystal display capable of presenting a transmission type display using a backlight and a reflection type display using ambient light.
For instance, JP-A 9-318807 discloses to use a cholestric liquid crystal-dispersed cholesteric film as a color filter. This color filter is superior in luminance and color purity to conventional color filters using pigments or dyes. Another advantage of the color filter is that by use of a ultraviolet curing type cholesteric liquid crystal in which the wavelength of its selectively reflected light changes with temperature and a chiral agent ratio and which can be cured by ultraviolet irradiation while its liquid crystal state is maintained, it can be fabricated more easily as compared with the prior art.
Such a cholesteric liquid crystal as mentioned above, and a chiral nematic liquid crystal is characterized by having a long-range order of liquid crystal molecule axis as well as a spatially helically varying a director. In a plane parallel with the liquid crystal molecule axis, namely the liquid crystal has an orientational order similar to that of a nematic phase. In the adjacent plane, however, this local orientational direction is slightly rotated. This successively occurs, giving a helical structure.
On the other hand, natural light may be broken down into dextrorotatory polarization and levorotatory polarization. The chloresteric or chiral nematic liquid crystal has such properties that when both dextrorotatory component light and levorotatory component light are incident parallel with the helical axis of the liquid crystal, only circularly polarized light in the same rotatory direction as the twist direction of that liquid crystal is reflected thereat whereas the other circularly polarized light component passes therethrough.
Since there is then no change in the phase of the reflected light with respect to the incident light, the direction of polarization of the reflected light before and after reflection is kept invariable, and the wavelength of the reflected light changes with the twist pitch of the cholesteric or chiral nematic liquid crystal. This pitch changes with the amount of the chiral agent added, which induces twist force in the liquid crystal, and suitable external fields (e.g., temperature, electric, magnetic or other fields).
By control of such parameters as mentioned above in the visible region, it is thus possible to form red, green and blue transmitted light and reflect light other than the transmitted light toward a light source side for reuse, thereby setting luminance at a level higher than could be achieved with a conventional color filter.
By the way, JP-A's 8-320480, 8-234196 and 2000-347179 show a transmission type color liquid crystal display wherein R (red), G (green) and B (blue) pixels are provided on the backlight side with a cholesteric liquid crystal filter that transmits light in the R, G and B wavelength ranges and reflects light in other wavelength ranges, and on the viewing side with an absorption type color filter that transmits only light in the R, G and B wavelength ranges and absorbs light in other wavelength ranges, thereby improving on the efficiency of use of backlight and eliminating influences of ambient light.
Further, JP-A 2000-193962 discloses an arrangement wherein a semitransparent type cholesteric liquid crystal filter, i.e., a semitransparent type cholesteric liquid crystal filter designed to transmit a specific proportion, e.g., 10%, rather than 100%, of specific circularly polarized light in a specific wavelength range is located on the side of the liquid crystal layer of a liquid crystal display device opposite to the viewing side, so that for a reflection type display, a specific circularly polarized light component of ambient light is reflected at this semitransparent type cholesteric liquid crystal filter, and for a transmission type display, for instance, just 10% of a specific circularly polarized light component of backlight are transmitted therethrough. On the other hand, JP-A 2001-4843 discloses that such a semitransparent type cholesteric liquid crystal filter is constructed of a triple-layer cholesteric liquid crystal filter arrangement for reflecting light in the R, G and B wavelength ranges at a given proportion.
Furthermore, JP-A 2000-131684 discloses an arrangement wherein color filters of the absorption type are located on both the viewing side and the backlight side of the liquid crystal layer of a liquid crystal display device with a cholesteric liquid crystal filter interposed between the backlight side-color filter of the absorption type and the liquid crystal layer, so that whether for reflection type display purposes or for transmission type display purposes, illumination light is transmitted twice through the absorption type of color filters, thereby ensuring enough color density in either case.